Imbalance in truck or car wheel assemblies (tire+rim) due to imperfect tires or rims causes vibrations which, apart from the discomfort to the driver and any passengers, can dramatically increase the wear rate of the tire as well as mechanical wear on e.g. bearings, etc. Normally, two types of imbalance are to be distinguished between: static imbalance, caused by uneven weight distribution (also known as "heavy spots") in the rotational plane of the wheel assembly, and dynamic imbalance, caused by uneven weight distribution in relation to the rotational plane of the wheel assembly. These two types of imbalance are schematically illustrated in FIGS. 5 and 6 on the drawing. If static imbalance is present, shock waves (vibrations) will be generated each time the heavy spot contacts the road surface during driving, whereas dynamic imbalance causes a wobbling of the wheel which in turn causes vibrations.
Normally, such imbalances are remedied by fastening lead weights on the wheel rim (cf. FIGS. 5 and 6). However, this balancing technique, being of a fixed nature, is not able to compensate for changes in load, uneven wear of the tires or dirt collection on the rim (both leading to changes in weight distribution) and the like. Therefore, the balancing by means of lead weights should be repeated several times during the lifetime of a tire. However, as it will be evident, such a rebalancing procedure is troublesome and may therefore often be dispensed with. Furthermore, in particular on heavy trucks, twin wheels on non-steering axles are even more difficult to rebalance due to i.a. inaccessibility, and such rebalancing or, indeed, even initial balancing is therefore often dispensed with on such wheels, and only a balancing of wheels with a steering function is carried out. As noted above, the consequent imbalances will in effect lead to shorter effective lifetime of the tire. Therefore, it is obvious that a serious need exists for an improved method and improved means for balancing motor vehicle wheels.
The present invention is based on the discovery that the vibrations caused by imbalance in a wheel assembly can induce motion of a liquid inside the tire away from the vibration source (the above-mentioned heavy spot). However, utilizing this "vibrational pressure" to achieve balancing of a wheel assembly (i.e. the gravitational center is in the intersection between the rotational plane and the axis of rotation) is by no means trivial. If one only introduced a free-flowing liquid such as water into a standard 22" (56 cm) truck tire spinning without loading at a rate corresponding to 90 km/h (i.e. approximately 7 revolutions per second), the liquid would merely distribute itself evenly around the inner rim of the tire due to the centrifugal force (which is in the order of 125.times.g) in such a way that the surface of the liquid is always at a constant distance from the rotational axis. Since such a distribution is unable to take into account any uneven distribution of weight, this procedure might just as well aggravate as diminish or leave unchanged any existing imbalance. If for example imbalance was introduced on this wheel assembly by attaching a lead weight on the rim, the imbalance would evidently not affect the centrifugal force exerted on the liquid and would therefore not affect the distribution of the liquid. If a load was applied to this wheel assembly by means of spinning it against a metal drum (in order to simulate actual road conditions), strong vibrations would occur each time a heavy spot hit the drum. As mentioned earlier, such vibrations would result in a "vibrational pressure" on the liquid directed away from the heavy spot. In a free-flowing liquid (i.e. having a short relaxation time), however, this movement due to the vibrations would be counteracted by the centrifugal force, and the net result would merely be an oscillating movement of the liquid with no balancing effect whatsoever.